The present invention relates generally to electrically conductive gels which are used to transmit an electrical signal between the human skin and an electrode attached to an electrical recording or stimulating device.
Frequently in the practice of medicine it is desirable to make electrical contact with the body. Such contact may be for the purpose of measuring electrical signals, as in the making of electrocardiograms or electroencephalograms, or applying electrical impulses to the body during electrotherapy.
The skin is a difficult structure with which to make reliable, low resistance, electrical contact. Accordingly, it has become customary in the art to utilize a conductive medium between the electrode and the skin to enhance conductivity. This medium normally takes the form of a conductive paste or gel which makes intimate contact with the skin, by conforming to the contours of the skin, and fills the gaps between the skin and the electrode, thus providing a more reliable path for the electrical current than is afforded by dry surface contact between electrode and skin. These gels or pastes are normally made of a thickened aqueous mixture containing a conductive salt, such as sodium chloride. Conventional thickening agents typically include polymers, such as polyvinylalcohol (commonly referred to as PVA), polyethylene glycol or polypropylene glycol; glycerol and glycerol derivatives, such as glycerol monostearate; and a number of naturally occurring gummy materials, such as gum tragacanth, sodium alginate, locust bean gum and guar gum. A number of synthetic gummy materials and thickeners have also been used, including carboxymethyl cellulose, and proprietary materials such as Ganatrez materials sold by General Aniline and Film Corporation and Carbopols sold by the B. F. Goodrich Co.
Examples of the gels or pastes of the prior art can be found in U.S. Pat. Nos. 4,016,869; 3,998,215; 3,989,050; 3,658,726; and 3,265,638. These gels and creams are comprised of a thickened aqueous mixture and a salt or polarizing substance and do a reasonably effective job of making electrical contact with the skin. In particular, they make possible a contact which is largely free of voids and areas of poor or intermittent contact, which, when present, result in the generation of spurious electrical signals. Such spurious signals interfere with the collection of desired electrical data. However, all of these gels have one major disadvantage. They are sticky, messy materials which are unpleasant to use and are hard to remove from surfaces they have contacted. This problem has been addressed in the art by reinforcing the gelatinous or creamy conductive materials with porous or fibrous substances, which help to contain the gel or cream in a cohesive matrix, see U.S. Pat. No. 3,998,215. These structures, often referred to as gel pads, function well in regard to making good electrical contact with skin. However, the addition of nonconductive structural members within the conductive gel inevitably alters the resistance of a gel pad relative to that of the pure gel.
Germam Offenlegangschrift No. 27 27 396 discloses a viscoelastic conductive gel comprising a high molecular weight polysaccharide and a polyol, which is said to leave behind no residue on the skin. The gels disclosed therein are not crosslinked, have a low water content, are capable of carrying little salt and require the use of high molecular weight (at least about 10.sup.6) polysaccharides in order to provide the necessary cohesivity to be removed without leaving a residue. The low water content of these gels and their consequent inability to tolerate high salt levels limits their conductivity and sensitivity to electrical stimuli.
The gels of the present invention are an improvement over prior art gels. They maintain themselves as a cohesive mass without the need for mechanical reinforcement. They do not leave a residue on the skin or the electrode. Furthermore, they are capable of tolerating high concentrations of salt without breakdown of the gel. The gels of the present invention are less expensive to produce than the gels of the prior art since they can contain relatively less thickener and more water while still maintaining sufficient cohesive strength.